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Roughly one person in two is concerned about man-made global warming. Why? Because vivid, alarming forecasts are persuasive only for a time. We've seen it all before. Take the alarm over mercury in fish.

In 2004, a United States Environmental Protection Agency employee warned that each year 630,000 babies were born at risk of brain and nervous system damage due to ''unsafe'' levels of mercury in their mothers' blood.

It turns out that ''at risk'' meant having concentrations of mercury in the blood that were higher than unjustifiably low levels.

The Japanese consume a lot of fish and those ''at risk'' levels are exceeded by 74 per cent of Japanese women of childbearing age and by centuries-old mummified remains.

Yet, there is no evidence that Japanese are mentally deficient, and only benefits have been reported from high levels of fish consumption, including good brain function and improved intelligence at age four.
The alarming forecast of harm from mercury in fish was derived by extrapolating known bad effects from high doses of mercury to predict toxic effects from even very low levels, without bothering to check for evidence.

Because of the alarm, expectant mothers and others were discouraged from eating fish, to the detriment of their and their children's health. Bad forecast. Costly alarm.

With my colleague Scott Armstrong and suggestions from diverse experts, I identified 26 alarms that are analogous to the dangerous man-made global warming alarm.

You can find them and the mercury analogy on the PublicPolicyForecasting.com website. We were surprised to find so many. It seems alarms based on unscientific forecasts are a common social phenomenon.

All of the alarms turned out to be false, either completely or to such an extent that actions intended to be remedial caused much greater harm than the supposed problem.

When alarming forecasts are presented in the form of vivid scenarios seemingly realistic stories about the future people tend to ignore the low likelihood that they will come to pass. They want action.

This is especially so if they think the cost to them of action will be low, and if they can blame other people.

In modern times, when we are safer than we have ever been, clever and creative people become rich and famous by exploiting the public and media weakness for alarming scenarios, causing us to bounce around this way and that like frightened rabbits.

There is a recipe for persuasion in such cases, and fear induced by scary scenarios is part of it.

As Professor Stephen Schneider explained in making the case for global warming alarmism: ''So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have.''

Policy responses to the global warming and other environmental alarms are promoted in terms of social causes such as ''caring for the planet'' or ''caring for our children'', which has the intended effect of deflecting hard questions about the substance of these claims, and of demonising people who ask them.

It isn't immediately obvious that beneath the caring veneer are people who stand to gain from promoting the alarm, by advancing their careers and fortunes.

On the other side, the costs are diffused across many people and over time. But that doesn't make the costs any less real as people are forced to cut back on heating and cooling and generally reduce their living standards.

As time passes and policies such as New Zealand's emissions trading scheme are enacted, people come to realise that they will bear substantive costs as a consequence of policies enacted with the avowed purpose of combating ''dangerous global warming''.

People already face harder times with the current economic recession, and they hear scientists and politicians voice scepticism about the alarmist forecasts.

Observers are exposed to hints that there are glaring uncertainties and major disagreements in climate science.

Increasingly, people look for evidence.

When people become more involved in the issue, the persuasion recipe that initially works for alarmists breaks down.

People become less persuaded by appeals to trust the authorities, less susceptible to fear, less willing to accept emotional appeals from celebrities, less gullible. And this shows in the polls.

As have the other environmental alarms before it, the dangerous man-made global warming alarm will fade. It is already beginning to do so.

Unfortunately, policies that make most people worse off have been and will be implemented. Some at least of these polices are likely to continue indefinitely.

The news media will, by and large, devote little energy to reporting that the alarm was false, leaving people still feeling guilty that they are somehow causing harm as they go about their lives. Such are the forecasts from a structured analysis of analogies.